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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
PowerPoint® Lecture Presentations for
BiologyEighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Chapter 48
Neurons, Synapses, and Signaling
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Key concepts
1. Excitable membrane of neuronal
cells makes signaling possible.
2. Synapse is the fundamental unit of
information processing in the
nervous system.
Fig. 48-1
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: Lines of Communication
• Neurons are nerve cells that transfer
information within the body
• Neurons use two types of signals to
communicate: electrical signals (long-distance)
and chemical signals (short-distance)
Fig. 48-2
Nerveswith giant axonsGanglia
MantleEye
Brain
Arm
Nerve
The squid possesses
extremely large nerve
cells and is a good
model for studying
neuron function
Hodgkin, Huxley and Ionic Basis of Action Potential
Recording electrode inside squid giant axon
Resting potential and action potential recorded
between inside and outside of the axon with
capillary filled with sea water (1939)
Alan Hodgkin Andrew Huxley
Fig. 48-3
Sensor
Sensory input
Integration
Effector
Motor output
Peripheral nervous
system (PNS)
Central nervous
system (CNS)
Summary of information processing
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Neuron Structure and Function
• Most of a neuron’s organelles are in the cell
body
• Most neurons have dendrites, highly branched
extensions that receive signals from other
neurons
• The axon is typically a much longer extension
that transmits signals to other cells at synapses
• An axon joins the cell body at the axon hillock
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• A synapse is a junction between an axon and
another cell
• The synaptic terminal of one axon passes
information across the synapse in the form of
chemical messengers called
neurotransmitters
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Information is transmitted from a presynaptic
cell (a neuron) to a postsynaptic cell (a
neuron, muscle, or gland cell)
• Most neurons are nourished or insulated by
cells called glia
Fig. 48-4
Dendrites
Stimulus
Nucleus
Cellbody
Axonhillock
Presynapticcell
Axon
Synaptic terminals
Synapse
Postsynaptic cell
Neurotransmitter
Neuron structure and organization
Soma = 10 um
Axon length = 1 m (1,000,000 um)
Ratio = 1:100000
Fig. 48-5
Dendrites
Axon
Cellbody
Sensory neuron Interneurons
Portion of axon Cell bodies of
overlapping neurons
80 µm
Motor neuron
Structural diversity of neurons
Ramon y Cajal’s illustration of the
circuits in the hippocampus
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 48.2: Ion pumps and ion channels maintain the resting potential of a neuron
• Every cell has a voltage (difference in electrical
charge) across its plasma membrane called a
membrane potential
• Messages are transmitted as changes in
membrane potential (“excitable”)
• The resting potential is the membrane
potential of a neuron not sending signals
Fig. 48-6
OUTSIDE
CELL[K+]5 mM
[Na+]150 mM
[Cl–]120 mM
INSIDE
CELL[K+]
140 mM[Na+]15 mM
[Cl–]10 mM
[A–]100 mM
(a) (b)
OUTSIDE
CELL
Na+Key
K+
Sodium-potassiumpump
Potassiumchannel
Sodiumchannel
INSIDE
CELL
The basis of the membrane potential
Fig. 48-7
Innerchamber
Outerchamber
–90 mV
140 mM 5 mM
KCIKCI
K+
Cl–
Potassiumchannel
(a) Membrane selectively permeable to K+ (b) Membrane selectively permeable to Na+
+62 mV
15 mMNaCI
Cl–
150 mM
NaCI
Na+
Sodiumchannel
EK = 62 mV(log 5 mM140 mM ) = –90 mV ENa = 62 mV (log 150 mM15 mM = +62 mV)
Modeling a mammalian neuron
Fig. 48-8
TECHNIQUE
Microelectrode
Voltagerecorder
Referenceelectrode
Intracellular recording
Fig. 48-9
Stimuli
+50 +50
Stimuli
00
Mem
bra
ne p
ote
nti
al (m
V)
Mem
bra
ne p
ote
nti
al (m
V)
–50 –50Threshold Threshold
Restingpotential
Restingpotential
Hyperpolarizations–100 –100
0 1 2 3 4 5
Time (msec)
(a) Graded hyperpolarizations
Time (msec)
(b) Graded depolarizations
Depolarizations
0 1 2 3 4 5
Strong depolarizing stimulus
+50
0
Mem
bra
ne p
ote
nti
al (m
V)
–50 Threshold
Restingpotential
–100
Time (msec)
0 1 2 3 4 5 6
(c) Action potential
Actionpotential
Graded potentials and an action
potential in a neuron
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• An action potential occurs if a stimulus causes
the membrane voltage to cross a particular
threshold
• An action potential is a brief all-or-none
depolarization of a neuron’s plasma membrane
• Action potentials are signals that carry
information along axons (“firing frequency”)
slowly
more quickly
very rapidly
Some of Adrian’s first recordings from a very small number of
nerve fibers in the sensory nerves of cat’s toe (1926).
Adrian’s Laws: 1. The nerve impulse (action potential) is “all-or-none”
2. The strength of stimulus is coded by the firing frequency
Fig. 48-10-5
KeyNa+
K+
+50
Actionpotential
Threshold
0
1
4
51
–50
Resting potential
Mem
bra
ne p
ote
nti
al
(mV
)
–100Time
Extracellular fluid
Plasmamembrane
Cytosol
Inactivation loop
Resting state
Sodiumchannel
Potassiumchannel
Depolarization
Rising phase of the action potential Falling phase of the action potential
5 Undershoot
2
3
2
1
3 4
voltage-gated
ion channels
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• During the refractory period after an action
potential, a second action potential cannot be
initiated
• The refractory period is a result of a temporary
inactivation of the Na+ channels
Fig. 48-11-3
Axon
Plasma
membrane
Cytosol
Actionpotential
Na+
Actionpotential
Na+
K+
K+
ActionpotentialK+
K+
Na+
Conduction of an action potential
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Conduction Speed
• The speed of an action potential increases with
the axon’s diameter
• In vertebrates, axons are insulated by a myelin
sheath, which causes an action potential’s
speed to increase
• Myelin sheaths are made by glia—
oligodendrocytes in the CNS and Schwann
cells in the PNS
Fig. 48-12
Axon
Schwanncell
Myelin sheathNodes ofRanvier
Node of Ranvier
Schwanncell
Nucleus ofSchwann cell
Layers of myelin
Axon
0.1 µm
Schwann cells and the myelin sheath
Fig. 48-13
Cell body
Schwann cell
Depolarized region(node of Ranvier)
Myelinsheath
Axon
Saltatory conduction
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 48.4: Neurons communicate with other cells at synapses
• At electrical synapses, the electrical current
flows across the gap junction
• At chemical synapses, a chemical
neurotransmitter carries information from one
neuron to another
• Most synapses are chemical synapses
Fig. 48-14
Postsynapticneuron
Synapticterminalsof pre-synapticneurons
5 µ
m
Synaptic terminals on the cell body
of a postsynaptic neuron
Fig. 48-15
Voltage-gatedCa2+ channel
Ca2+12
3
4
Synapticcleft
Ligand-gatedion channels
Postsynapticmembrane
Presynapticmembrane
Synaptic vesiclescontainingneurotransmitter
5
6
K+Na+
A chemical synapse
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Generation of Postsynaptic Potentials
• Direct synaptic transmission involves binding of
neurotransmitters to ligand-gated ion channels
in the postsynaptic cell
• Neurotransmitter binding causes ion channels
to open, generating a postsynaptic potential
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Postsynaptic potentials fall into two categories:
– Excitatory postsynaptic potentials (EPSPs)
are depolarizations that bring the membrane
potential toward threshold
– Inhibitory postsynaptic potentials (IPSPs)
are hyperpolarizations that move the
membrane potential farther from threshold
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• After release, the neurotransmitter
– May diffuse out of the synaptic cleft
– May be taken up by surrounding cells
– May be degraded by enzymes
Fig. 48-16
Terminal branch
of presynaptic
neuron
E1
E2
I
Postsynaptic
neuron
Threshold of axon of
postsynaptic neuron
Resting
potential
E1 E1
0
–70
Mem
bra
ne p
ote
nti
al (m
V)
(a) Subthreshold, nosummation
(b) Temporal summation
E1 E1
Action
potential
I
Axon
hillock
E1
E2 E2
E1
I
Action
potential
E1 + E2
(c) Spatial summation
I
E1 E1 + I
(d) Spatial summation
of EPSP and IPSP
E2
E1
I
Summation of postsynaptic potentials
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Neurotransmitters
• There are five major classes of
neurotransmitters: acetylcholine, biogenic
amines, amino acids, neuropeptides, and
gases
Table 48-1
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Neuropeptides
• Neuropeptides include substance P and
endorphins, which both affect our perception
of pain
• Opiates bind to the same receptors as
endorphins and can be used as painkillers
Fig. 48-17
EXPERIMENT
RESULTS
Radioactive
naloxone
Drug
Protein
mixture
Proteins
trapped
on filter
Measure naloxone
bound to proteins
on each filter
Does the brain have a specific
receptor for opiates?
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
You should now be able to:
1. Distinguish among the following sets of terms:
sensory neurons, interneurons, and motor
neurons; membrane potential and resting
potential; ungated and gated ion channels;
electrical synapse and chemical synapse;
EPSP and IPSP; temporal and spatial
summation
2. Explain the role of the sodium-potassium
pump in maintaining the resting potential
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
3. Describe the stages of an action potential;
explain the role of voltage-gated ion channels
in this process
4. Explain why the action potential cannot travel
back toward the cell body
5. Describe saltatory conduction
6. Describe the events that lead to the release of
neurotransmitters into the synaptic cleft
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
7. Explain the statement: “Unlike action potentials, which are all-or-none events, postsynaptic potentials are graded”
8. Name and describe five categories of neurotransmitters